Fused NHase with improved specific activity and stability

ABSTRACT

The present invention provides a fused NHase with improved specific activity and stability, which relates to the field of genetic engineering. This invention provides a method of overexpressing a fused NHase in  E. coli  and producing a mutant NHase with improved stability and product tolerance. The invention provides a simple, efficient and safe method of making mutant NHase, and can produce a large amount of soluble NHases in a short period. The present invention makes a contribution to large-scale industrial production and further theoretical study of NHases.

CROSS-REFERENCES AND RELATED APPLICATIONS

This application claims the benefit of priority to Chinese ApplicationNo. 201510195795.0, entitled “A fused NHase with improved specificactivity and stability”, filed Apr. 22, 2015, which is hereinincorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to the field of genetic engineering, andmore particularly relates to a fused NHase with improved specificactivity and stability.

Description of the Related Art

Nitrile hydratase (NHase; EC 4.2.1.84) is an enzyme catalyzing thehydration of a broad scope of nitriles to the corresponding amides. TheNHase comprises a β-subunit, a α-subunit and a regulatory subunit and itis generally divided into the cobalt-type (Co-NHase) or the iron type(Fe-NHase) depending on the metal ion chelated with the active site.

NHase has been widely used in the industrial production of highlypurified acrylamide and nicotinamide, since biotechnology synthesis hasadvantages of low-cost, low-energy consumption and less pollutioncompared to traditional chemical synthesis. However, most NHases withhigh activity are unstable during industrial application. For example,the NHases of Pseudomonas chlororaphils B23 and Rhodococcus sp. N-774are unstable above 20° C., and the NHase of Rhodococcus rhodochrous J1is merely stable between 10° C. and 30° C. In addition, it is necessaryto maintain low reaction temperature to stabilize the NHases byrefrigeration because of the exothermic reaction of nitrile-hydration,which usually causes enormous redundant energy cost. Furthermore,tolerance of NHase to high concentrations of the product is necessary inindustrial manufacturing. Therefore, a more stable NHase with highactivity and high tolerance is required for industrial manufacturing.

DETAILED DESCRIPTION

To solve the problems described above, the present invention provides amethod of improving the specific activity, stability and tolerance ofNHase. Usually, the subunits of NHase are separated, and they would bedepolymerized at high temperatures which could result in enzymeinactivation. Therefore, the present invention fuses the β- andα-subunits with covalent bonds through molecular approaches, whicheliminates the possibility of subunits depolymerization. The resultedfused NHase with improved stability is more suitable for using in theindustrial production of acrylamide and the fusion strategy could beapplicable for different NHases with separated subunits.

The present invention provides a mutant NHase with improved specificactivity and stability. The β- and α-subunits of the mutant NHase arefused in the mutant NHase, and the regulatory subunit is either fused orcoexpressed with the fused α- and β-subunits.

In one embodiment of the present invention, the nucleotide sequence ofthe mutant NHase from 5′ to 3′ is β-subunit gene (B gene), α-subunitgene (A gene), and regulatory subunit gene (P14K gene) fused together.

In one embodiment of the present invention, the mutant NHase isreconstructed from the parent NHase whose nucleotide sequence is SEQ IDNO: 1. And the amino acid sequences of the α-subunit, β-subunit andregulatory subunit of the mutant NHase are the same as those of theparent NHase from Pseudomonas putida NRRL-18668.

In one embodiment of the present invention, the amino acid sequenceswhich encode the β-subunit, α-subunit and regulatory subunit are SEQ IDNO: 2, SEQ ID NO: 3 and SEQ ID NO: 4, respectively.

The B gene and A gene are linked by a linker, and the nucleotidesequence of the linker is SEQ ID NO: 5.

In one embodiment of the present invention, the nucleotide sequence ofthe mutant NHase is SEQ ID NO.6 or SEQ ID NO.7.

The present invention also provides plasmids containing the amino acidsequences of the mutant NHase above and genetically engineered strainsexpressing the mutant NHase.

In one embodiment of the present invention, the genetically engineeredstrain is a recombinant E. coli BL21 (DE3).

The present invention also provides a method of constructing agenetically engineered strain expressing the mutant NHase.

In one embodiment of the present invention, the method of constructingthe genetically engineered strain comprises cloning the nucleotidesequence shown in SEQ ID NO: 6 or SEQ ID NO: 7 to the expression plasmidof pET-28a to make a recombinant plasmid and transforming therecombinant plasmid into E. coli BL21(DE3).

The present invention also provides a method of producing NHases by thegenetically engineered strain. The recombinant E. coli expressing themutant NHase was cultivated in 2YT medium (tryptone 16 g/L, yeastextract 10 g/L, NaCl 5 g/L) at 37° C. When the optical density at 600 nm(OD₆₀₀) of the culture reached 0.8, isopropyl-D-1-thiogalactoside (IPTG)and CoCl₂.6H₂O were added to the medium to induce the expression andmaturization of NHase. The culture was subsequently incubated at 24° C.for 16 h.

The present invention also provides a method of improving the specificactivity and stability of NHase, wherein the NHase is made by fusing theB and A gene together and coexpress the P14K gene, or by fusing the B, Aand P14K gene together.

In one embodiment, the present invention provides a method to fuse theB, A and P14K gene from 5′ to 3′ in the order of “B gene, A gene, P14Kgene”, and the B and A gene are connected by a linker whose nucleotidesequence is set forth in SEQ ID NO: 5.

The application of the mutant NHase, especially the application of themutant NHase in acrylamide production is also under the scope of thepresent invention.

The mutant NHases obtained by the gene fusion strategy of the presentinvention exhibited significantly improved specific activity,thermostability and product tolerance than those of the wild type NHase.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1. SDS-PAGE of the wild type NHases and the fused NHases expressedby E. coli. Line 1, molecular weight marker; line 2, the wild typeNHases; line 3, the mutant NHase-(BA); line 4, the mutantNHase-(BA)P14K; line 5, the mutant NHase-(BAP14K).

FIG. 2. Half-time in 50° C. of the wild type NHase and the fused NHases.

FIG. 3. Product tolerance of the wild type NHase and the fused NHases.

FIG. 4. The optimal pH of the wild type NHase and the fused NHases.

EXAMPLES

Materials and Methods:

2YT medium: 16 g·L⁻¹ tryptone, 10 g·L⁻¹ yeast extract, 5 g·L⁻¹ NaCl.

The activity of NHase was detected by the method described as follows.The reaction mixture contained 500 μL 200 mM 3-cyanopyridine and 10 μlof the appropriate amount of the enzyme solution. The reaction wasperformed at 25° C. for 10 min and terminated with the addition of 500μL of acetonitrile. Then the supernatant was collected by centrifugationand filtered though a 0.22 μm pore-size filter before measured by HPLC.One unit (U) of NHase activity is defined as the amount of enzyme thatreleased 1 μmol nicotinamide per min under these assay conditions.

HPLC conditions: the mobile phase was water-acetonitrile buffer;detection wavelength was 215 nm; the column was C18 column.

Example 1 Construction of the Recombinant E. coli Expressing the WildType NHases-BAP14K

Construction of the recombinant E. coli expressing the wild typeNHases-BAP14K was carried out by the following steps:

(1) Amplification of the parent NHase gene: Primers were designedaccording to the published sequence in NCBI to amplify the ABP14K geneencoding the parent NHase from P. Putida. The amino acid sequence of theβ-subunit, α-subunit and regulatory subunit of the parent NHases wereSEQ ID NO: 2, SEQ ID NO: 3 and SEQ ID NO: 4, respectively.

(2) Construction of recombinant plasmid containing the ABP14K gene: theamplified DNA fragment of step 1 was digested with Nde I and Hind III,and then ligated into the Nde I and Hind III sites of pET-24a to createa recombinant plasmid containing the ABP14K gene. The recombinantplasmid was named pET-24a-ABP14K.

(3) Construction of recombinant plasmid containing the full-lengthBAP14K gene: pET-24a-ABP14K was used as a template. B gene was amplifiedby primer pairs B-up (SEQ ID NO: 8) and B-down(BA) (SEQ ID NO: 9), Agene was amplified by primer pairs A-up(BA) (SEQ ID NO: 10) andA-down(AP) (SEQ ID NO: 11), and the P14K gene was amplified by primerpairs P14K-up(AP) (SEQ ID NO: 12) and P-down (SEQ ID NO: 13). The sameamount of B, A, P gene were used as templates and the full-length BAP14Kgene was amplified by an overlap extension PCR protocol with primerpairs B-up (SEQ ID NO: 8) and P-down (SEQ ID NO: 13). The recombinantplasmid containing the BAP14K gene was named pET-24a-BAP14K, and themutant NHase expressed by pET-24a-BAP14K was defined as the wild typeNHase.

(4) Transformation of pET-24a-BAP14K into E. coli BL21 (DE3): Therecombinant plasmid pET-24a-BAP14K was transformed into E. coli BL21(DE3). The positive transformants expressing the wild type NHase werescreened.

Primers used in the present invention were shown in Tab. 1.

TABLE 1 Primers SEQ ID Primer sequence (5′ to 3′) NO B-up GGAATTC 

AATG 8 GCATTCACGATACT B-down CATATCTATATCTCCTTTC 9 (BA)ACGCTGGCTCCAGGTAGTC A-up TGAAAGGAGATATAGATAT 10 (BA) GGGGCAATCACACACGCA-down CATATCTATATCTCCTTTT 11 (AP) AATGAGATGGGGTGGGTT P14K-upTAAAAGGAGATATAGATAT 12 (AP) GAAAGACGAACGGTTTC P-down CCG 

TCAAGCCAT 13 TGCGGCAACGA B-Nde I- GGAATTC 

AATGG 14 up CATTCACGATAC P-Hind GCCC 

TCAAGCCA 15 III-down TTGCGGCAACGA A-Hind GCCC 

TCAATGAG 16 III-down ATGGGGTGGGTT Linker1- TACCTGGAGCCAGCGCCAGG 17 upTGGGCAATCACACACGCAT Linker1- CGTGTGTGATTGCCCACCTG 18 downGCGCTGGCTCCAGGTAGTC Linker2- CCCACCCCATCTCATCCAAA 19 up TGGAGATATAGATATGLinker2- CATATCTATATCTCCATTTG 20 down GATGAGATGGGGTGGG Note: restrictionsites were in italics and bold; overlapping sequences were underlined.

Example 2. Construction of the Recombinant E. coli Expressing theNHase-(BA)P14K

The recombinant E. coli expressing the NHase-(BA)P14K was constructed bythe following steps:

(1) The B and A gene were fused by linker 1 (SEQ ID NO: 5) by primerpairs Linker1-up (SEQ ID NO: 17) and Linker1-down (SEQ ID NO: 18) usingpET-24a-BAP14K as a template. The resulted pET-24a-(BA)P14K was used asa template to amplify the (BA)P14K gene by primer pairs B-Nde I-up (SEQID NO: 14) and P-Hind III-down (SEQ ID NO: 15). The amplified (BA)P14Kfragment was then digested with Nde I and Hind III, ligated into the NdeI and Hind III sites of pET-28a. The resulted recombinant plasmidpET-28a-(BA)P14K could express a fused NHase (nucleotide sequence shownin SEQ ID NO: 6), whose β- and α-subunits were fused together and theregulatory subunit was coexpressed. The NHase expressed bypET-28a-(BA)P14K was defined as NHase-(BA)P14K.

(2) The recombinant plasmid pET-28a-(BA)P14K was transformed into E.coli BL21 (DE3). Positive transformants expressing the NHase-(BA)P14Kwere screened.

Example 3. Construction of the Recombinant E. coli Expressing theNHase-(BAP14K)

The recombinant E. coli expressing the NHase-(BAP14K) was constructed bythe following steps:

Primer pairs Linker2-up (SEQ ID NO: 19) and Linker2-down (SEQ ID NO: 20)were used to connect the A gene and P14K gene and pET-28a-(BA)P14K wasthe template. The resulted plasmid pET-28a-(BAP14K) contained a fusedNHase gene whose B, A, and P14K gene fragments were fused together(nucleotide sequence shown in SEQ ID NO: 7). The NHase expressed bypET-28a-(BAP14K) was defined as NHase-(BAP14K).

The recombinant plasmid pET-28a-(BAP14K) was transformed into E. coliBL21 (DE3). The positive transformants expressing the NHase-(BAP14K)were screened.

Example 4. Expression and Characterization of the NHases

The E. coli recombinants obtained in example 1-3 were used to expressthe NHases.

The E. coli recombinants were firstly cultivated in 10 ml of liquid 2YTmedium containing 50 μg/ml kanamycin at 37° C., then transferred to 500ml of liquid 2YT medium with 1% inoculation. When OD₆₀₀ of the culturereached 0.8, IPTG was added to a final concentration of 0.4 mM to induceNHase expression, and CoCl₂.6H₂O was added to a final concentration of0.05 g/l to obtain mature NHase. The culture was subsequently incubatedat 24° C. for 16 h and then the cells were harvested for SDS-PAGE.

Results indicated that the wild type NHase, NHase-(BA)P14K andNHase-(BAP14K) were successfully expressed, as shown in FIG. 1. The line3 represented the mutant NHase-(BA) whose β-subunit and α-subunit werejust fused in the absence of the regulatory subunit.

The characteristics of the subunits fused NHases:

Specific Activity

Determination of NHases was conducted by the following method. The E.coli recombinants were collected by centrifugation and resuspended witha 0.01M phosphate buffer (pH 7.5) twice before ultrasonic disruption.The enzyme in the supernatant was purified and then the enzyme activitywas detected by HPLC.

Compared with 324.8 U/mg of the wild type NHase, the specific activityof NHase-(BA)P14K and NHase-(BAP14K) were 499.2 U/mg and 452.5 U/mg,which were increased by 53.7% and 39.3%, respectively. In addition, thespecific activity of NHase-(BA) (FIG. 1, line 3) was 69.1 U/mg,indicating that the P14K was also necessary for cobalt incorporation inthe fused NHase.

Furthermore, the kinetic parameters (K_(m), V_(max), k_(cat) andk_(cat)/K_(m)) of NHase-(BA)P14K and NHase-(BAP14K) were compared withthe wild-type NHase. Results showed that the k_(cat) value ofNHase-(BA)P14K (723.4 s⁻¹) and NHase-(BAP14K) (676.5 s⁻¹) were bothapproximately 2-fold of the wild-type NHase (335.1 s⁻¹), indicating thatthe fused NHases exhibited faster catalyze rate. In addition, thek_(cat)/K_(m) value of NHase-(BA)P14K (11.8·10³ s⁻¹M⁻¹) was about 1.5fold of the wild-type NHase (8.1·10³ s⁻¹M⁻¹), indicating highercatalytic efficiency of NHase-(BA)P14K.

Thermostability

The thermostability of the NHases was measured by the following steps.First, the eppendorf tube containing the enzyme solution was placed in ametal bath at 50° C. for a while before placed on ice. And then, thetube was placed at 25° C. in the metal bath and 200 mM 3-cyanopyridine(substrate) was added to it. Ten minutes later, acetonitrile was addedto terminate the reaction.

As shown in FIG. 2, the half-life times of NHase-(BA)P14K andNHase-(BAP14K) were 26 min and 18 min, respectively, while that of thewild-type NHase was 9 min. Results suggested that the NHase-(BA)P14K andNHase-(BAP14K) exhibited higher thermostability than the wild-typeNHase.

Product Tolerance of the NHases

The product tolerance of the NHases was measured by the followingmethod. The reaction was conducted in 20 mM 3-cyanopyridine (substrate)with and without 0.5 M nicotinamide (product) for 10 min. The reductionof 3-cyanopyridine in each reaction was measured (FIG. 3), and thereduction ratio (the proportion of the reduced 3-cyanopyridine amount inthe reaction with and without 0.5 M nicotinamide) was calculated.

Results showed that the consumption of substrate of NHase-(BA)P14K andNHase-(BAP14K) in product containing reaction systems were increased by26% and 18%, respectively compared with the wild type NHase, andincreased by 23% and 15% respectively in reaction systems withoutproduct. In addition, the reduction ratios of NHase-(BA)P14K (0.86) andNHase-(BAP14K) (0.83) were higher than that of the wild type (0.80),indicating that the fused NHases exhibited stronger product tolerancethan that of the wild type.

The Optimum pH

The enzyme activities of the fused NHases were measured under differentpH and compared with the wild type, and the activity under theirrespective optimum pH was defined as 1(100%). As shown in FIG. 4, theoptimum pH of the three NHases were about 7.5.

These data showed that the specific activity, thermostability andproduct tolerance of NHase could be significantly increased by fusingthe β subunit and the α subunit with the regulatory subunit fused orcoexpressed at the same time.

While the present invention has been described in some detail forpurposes of clarity and understanding, one skilled in the art willappreciate that various changes in form and detail can be made withoutdeparting from the true scope of the invention. All figures, tables,appendices, patents, patent applications and publications, referred toabove, are hereby incorporated by reference.

What is claimed is:
 1. A mutant Nitrile hydratase (NHase) with improvedspecific activity and stability compared to its wild type NHase, whereinsaid mutant NHase comprises, from the N-terminus to the C-terminus, aβ-subunit of SEQ ID NO: 2 and a linker encoded by a nucleotide sequenceof SEQ ID NO: 5 fused to an α-subunit of SEQ ID NO: 3 in a singlepolypeptide, and a regulatory subunit of SEQ ID NO:
 4. 2. The mutantNHase of claim 1, wherein said mutant NHase is encoded by a nucleotidesequence of SEQ ID NO:
 6. 3. The mutant NHase of claim 1, wherein saidmutant NHase is expressed by a plasmid.
 4. The mutant NHase of claim 1,wherein said mutant NHase is expressed by a genetically engineeredstrain.
 5. The mutant NHase of claim 4, the wherein said geneticallyengineered strain is Escherichia coli.
 6. The mutant NHase of claim 4,the wherein said genetically engineered strain is constructed by thefollowing steps: a), cloning the nucleotide sequence of SEQ ID NO.6 tothe expression vector of pET-28a to create a recombinant plasmid; andb), transforming the recombinant plasmid into E. coli BL21.
 7. A methodof producing acrylamide, comprising using the mutant NHase of claim 1 tomake acrylamide.